1. Field of the Invention
This invention relates generally to the field of orthodontics. More particularly, the invention relates to orthodontic brackets and archwires for straightening the teeth of a patient.
2. Description of Related Art
A widely used method to straighten or align teeth of a patient is to bond brackets onto the teeth and run elastic wires of rectangular cross-sectional shape through the bracket slots. Typically, the brackets are off-the-shelf products. In most cases, they are adapted to a certain tooth (for instance an upper canine), but not to the individual tooth of a specific patient. The adaptation of the bracket to the individual tooth is performed by filling the gap between tooth surface and bracket surface with adhesive to thereby bond the bracket to the tooth such that the bracket slot, when the teeth are moved to a finish position, lies in flat horizontal plane. The driving force for moving the teeth to the desired finish position is provided by the archwire. For lingual brackets, a system has been developed by Thomas Creekmore that has vertical bracket slots. This allows an easier insertion of the wire. The longer side of the wire is therefore oriented vertically. Unitek has marketed this bracket system under the trade name CONSEAL.™
A computerized approach to orthodontics based on design and manufacture of customized brackets for an individual patient, and design and manufacture of a customized bracket placement jig and archwire, has been proposed in the art. See U.S. Pat. No. RE 35,169 to Lemchen et al. and U.S. patents to Andreiko et al., U.S. Pat. Nos. 5,447,432, 5,431,562 and 5,454,717. The system and method of Andreiko et al. is based on mathematical calculations of tooth finish position and desired ideal archform. The method of Andreiko et al. has not been widely adopted, and in fact has had little impact on the treatment of orthodontic patients since it was first proposed in the early 1990s. There are a variety of reasons for this, one of which is that the deterministic approach proposed by Andreiko et al. for calculating tooth finish positions does not take into account unpredictable events during the course of treatment. Furthermore, the proposed methods of Andreiko et al. essentially remove the orthodontist from the picture in terms of treatment planning, and attempt to replace his or her skill and judgment in determining tooth finish positions by empirical calculations of tooth finish positions.
Typically, the wires used in orthodontic treatment today are off-the-shelf products. If they need to be individualized by the orthodontist, the goal is to get along with as few modifications as possible. Therefore, the brackets are designed in a manner that at the end of treatment, when teeth are aligned, the bracket slots are supposed to be located and oriented in a planar manner. This means that a wire that would run passively through the slots, without applying any force, would be planar (flat). This treatment regime is known as “straight wire”. It dominates orthodontics worldwide. It is efficient for both manufacturers and the orthodontist. The customized orthodontic appliances proposed by Andreiko et al. call for a flat planar wire, but with the curvature in a horizontal plane customized for the individual and dictated by the shape of the ideal desired archform for the patient.
The so-called straight wire approach that continues to be used in orthodontics today has some noteworthy disadvantages in terms of patient comfort. The need to close the gap between the bracket bonding surface and the tooth surface with adhesive always leads to an increased overall thickness of the appliance. For brackets that are bonded labially, this is acceptable, as labial tooth surfaces are very uniform for different individuals, and the gap to be closed is not significant. However, lingual (inner) surfaces of teeth show a much greater variation among patients. To achieve the goal to orient the bracket in a manner such that the slot is parallel to all other slots when treatment is finished, the thickness of adhesive that is necessary often is in the range of 1 to 2 mm. It is obvious that every fraction of a mm added to appliance thickness significantly increases patient discomfort. Especially with lingual brackets (bracket bonded to the lingual surface of the teeth), articulation problems arise, and the tongue is severely irritated for several weeks after bonding. The tooth surfaces next to these adhesive pads are difficult to clean, thus serving as collecting point for bacteria and causing gingival inflammation. The further the archwire is away from the tooth surface, the more difficult it is to achieve a precise finishing position for each tooth. An error of only 10 degrees in torque (rotation around the wire axis) may well induce a vertical error in tooth position of more than 1 mm.
Another significant disadvantage of thick brackets, especially when bonding lingually, arises when the front teeth are severely crowded (which is often the cause for orthodontic treatment). Since the space is more restricted at the lingual surface due to the curvature of the jaw, not all brackets may be bonded at one session. Rather, the orthodontist has to wait until the crowding has decreased until all brackets may be placed. Crowding also creates problems for labial brackets. Geometrical considerations dictate that this constriction problem becomes worse as the thickness of the bracket/bracket bonding pad/adhesive combination increases.
Another problem in orthodontics is to determine the correct bracket position. At the time of bonding, teeth may be oriented far away from the desired position. So the task to locate the brackets in a manner that a flat planar archwire drives teeth to the correct position requires a lot of experience and visual imagination. The result is that at the end of treatment a lot of time is lost to perform necessary adjustments to either bracket position or wire shape. This problem can be solved by creating an ideal set-up, either virtually using 3D scan data of the dentition or physically by separating a dental model of the dentition into single teeth and setting up the teeth in a wax bed in an ideal position. The brackets can then be placed at this ideal set-up at optimal positions, in a manner that a flat wire running through the bracket slots would drive the teeth exactly into the ideal target. This again may be done virtually in a computer or physically. After this is done, the bracket position has to be transferred on a tooth-by-tooth basis into the maloccluded (initial) situation. Basing on this maloccluded situation, a transfer tray enveloping the brackets can be manufactured, which allows bonding the brackets exactly at the location as defined at the set-up. Such as technique is taught generally in Cohen, U.S. Pat. No. 3,738,005.
The published PCT patent application of OraMetrix, Inc., publication No. WO 01/80761, describes a wire-based approach to orthodontics based on generic brackets and a customized orthodontic archwire. The archwire can have complex twists and bends, and as such is not necessarily a flat planar wire. The entire contents of this document is incorporated by reference herein. This document also describes a scanning system for creating 3D virtual models of a dentition and an interactive, computerized treatment planning system based on the models of the scanned dentition. As part of the treatment planning, virtual brackets are placed on virtual teeth and the teeth moved to a desired position by a human operator exercising clinical judgment. The 3D virtual model of the dentition plus brackets in a malocclused condition is exported to a rapid prototyping device for manufacture of physical model of the dentition plus brackets. A bracket placement tray is molded over the model. Real brackets are placed into the transfer tray in the location of where the virtual brackets were placed. Indirect bonding of the brackets to the teeth occurs via the transfer tray. The system of WO 01/80761 overcomes many of the problems inherent in the Andreiko et al. method.
During the course of treatment, brackets may come off, for instance if the patient bites on hard pieces of food. Obviously, the transfer tray used for initial bonding will not fit any more as teeth have moved. While it is possible to cut the tray (such as described in WO 01/80761) into pieces and use just the one section that is assigned to the bracket that came off, to replace the bracket the reliability of this procedure is limited, as a small piece of elastic material is not adequate to securely position a bracket. It may therefore be required to create a new transfer tray adapted to the current tooth position using a costly lab process.